1. Field of the Invention
This invention relates in general to a diffusion barrier and separation substance for metal parts which adjoin each other in an oxygen-free atmosphere and in particular to a new and useful diffusion barrier and to a method of forming the barrier.
2. Description of the Prior Art
The present invention deals particularly with a diffusion barrier and a separation substance for metal parts which adjoin each other in an oxygen-free, inert, preferably heliumcontaining atmosphere such as used in a closed cycle high temperature reactor or gas turbine. Adhesion, friction and wear of metals which contact each other statically or in a sliding or rolling motion usually increases with the increasing temperature. The difficulties resulting therefrom can be kept within acceptable limits as long as the ambient atmosphere contains oxygen or an oxygen along with a water vapor and the interengaged materials are made of high temperature alloys forming stable oxides. Such oxide films on surfaces prevent an intimate contact between the metals. It is true that in this case also even with small loads very high specific surface pressures are produced on the surface peaks contacting each other, so that the surface peaks are plastically deformed. This causes the protective oxide films to be interrupted and locally limited welded areas may be formed. However due to the sliding motion, such weld bridges are sheared off. The broken metal surface areas are chemically highly active and in a surrounding oxidizing atmosphere they tend to coat themselves immediately with a new protective oxide film.
Such a healing process cannot take place however with the mating surfaces located in a vacuum or in an inert atmosphere. Such conditions result in very high frictional coefficients, with values much higher than one, and in a rapid destruction of the mating surfaces and with an intense wear thereof. If the contacting bodies do not move for long periods of time, alloyed structures may form over large surface areas by local welding and the adhesion forces may grow to values reaching the strength of the material itself.
This problem is particularly important in connection with the development of helium gas turbines and helium cooled high temperature reactors and becomes manifest, for example, through a premature wear and failure of bearings and a deformation and rupture of parts especially of pipes which can no longer freely move in accordance with the temperature gradient and also in such applications as diffusion welding of screw connections, flanges, buckets, fastening elements, etc.